Pulp making is carried out on large industrial scales worldwide to produce paper. Accordingly it is highly desirable that such pulp making operations be carried out in a cost effective, efficient operation with minimum manufacturing equipment downtime and minimum periods of reduced pulp making process equipment efficiency.
The basic steps in industrial pulp making are to convert plant fiber into chips, convert chips into pulp, wash the pulp, (optionally) bleach the pulp, and transform the pulp into suitable paper which can be used in a variety of paper products.
Typically, several chemical pulping processes are used in industrial pulp making operations. Well known industrial alkaline chemical pulping processes include the Kraft (or sulfate), soda and alkaline sulfite processes. The Kraft process makes the strongest fibers of any pulp producing process and is the most commonly used pulp making process in part due to its efficient recovery process for the cooking chemicals. While the present invention has applicability to any of the above alkaline chemical pulping processes, it is particularly useful with the Kraft process.
In the Kraft process, wood chips are digested to dissolve the lignin that holds the wood fibers together thereby producing clean fibers for further processing into a myriad of paper-based products. The digestion of the wood chips occurs in an alkaline solution mainly consisting of sodium hydroxide and sodium sulfide. As the process proceeds, the hydroxide becomes consumed and the sulfide slowly converts to hydroxide. The resulting pulp fibers are washed and removed leaving a solution, called black liquor, containing the lignin dissolved from the wood chips and residual hydroxide and sulfide. The black liquor is burned in a recovery boiler leaving a smelt primarily consisting of sulfide and sodium carbonate. This smelt is dissolved in water or “weak wash liquor” to produce green liquor. The objective of the remaining steps of the process is to convert sodium carbonate in the green liquor to sodium hydroxide so that the sodium hydroxide can be recycled and reused in the digesting process.
The reaction for converting the sodium carbonate to sodium hydroxide is often referred to as the “causticizing” process or reaction and is carried out in a “slaker” and a series of “causticizers,” to produce a white liquor that ideally has a high degree of sodium hydroxide and only a small amount of sodium carbonate. The causticizing reaction is controlled by the amount of lime (calcium oxide) introduced to the slaker and the flow rate of green liquor into the slaker. To produce white liquor having the appropriate characteristics, the flow rate of lime into the slaker is carefully regulated. It is essential to measure the characteristics of the green liquor and/or the white liquor in order to control the causticizing reaction.
Various types of sensor devices have been used to monitor and control the composition of green and white liquor. For example, the sensor device can comprise a series of electrodes embedded in housing mounted inside a digester or recausticizing tank. Unfortunately, calcium scales develop rapidly on the electrodes in such hostile environments resulting in measurement drift and loss of accuracy. The sensor devices must be physically cleaned to remove the scales. U.S. Pat. No. 6,235,123 to Millar describes a sensor device in which the electrodes are embedded in the surface of a housing that is equipped with a cleaning baffle. When cleaning is required, the cleaning baffle is rotated from a first position adjacent the electrodes, where the baffle does not interfere with contact between the liquor and the electrodes, to a second position, so that the baffle covers the electrodes from the liquor. Thereafter, a cleaning solvent is directed under pressure to the electrodes through a solvent channel. The spraying action is said to remove built-up materials on the electrodes. This in-situ cleaning technique has proven not to be effective in hostile environments in part because during the frequent cleaning cycles, the position of the baffles over the electrodes result in significant down times where the sensor device is not operating.